Recently, a hard disk device is incorporated in a personal computer, a notebook personal computer, and a DVD (Digital Versatile Disc) recording apparatus in order to record data. Particularly, in the hard disk device used in an apparatus such as the notebook personal computer based on portability, a magnetic disk in which a magnetic layer is provided on a glass substrate is used, and magnetic recording information is recorded in or read from a magnetic layer using a magnetic head (DFH (Dynamic Flying Height) head) that is slightly floated on a surface of the magnetic disk surface. A glass substrate is suitably used as the substrate for the magnetic disk because substrate is hardly plastically deformed compared with a metallic substrate.
The magnetic recording density is being increased in order to correspond to a demand for an increase of a storage capacity in the hard disk device. For example, a magnetic recording information area is finely formed using a perpendicular magnetic recording system in which a magnetization direction of the magnetic layer is oriented toward a direction perpendicular to the substrate surface, which allows the storage capacity to be increased in one disk substrate. In order to correspond to the further increase of the storage capacity, a floating distance of the magnetic head from the magnetic recording surface is extremely shortened to form the fine magnetic recording information area. In the substrate of the magnetic disk, the magnetic layer is formed flat such that the magnetization direction of the magnetic layer is oriented toward the direction substantially perpendicular to the substrate surface. Therefore, the glass substrate is formed such that surface irregularity of the glass substrate is decreased as much as possible.
The shortened floating distance of the magnetic head easily causes a head crush trouble or a thermal asperity trouble. Because these troubles are generated by the micro irregularity or a particle on the magnetic disk surface, the glass substrate is formed such that the surface irregularity in an end face is also decreased as much as possible in addition to the principal surface.
A press forming method and a floating method are well known as a method for manufacturing the sheet glass material that becomes a base of the glass substrate used in the magnetic disk.
For example, Japanese Patent No. 3709033 discloses a press forming method as the method for manufacturing the glass material that becomes the base of the glass substrate used in the magnetic disk. In the disclosed press forming method, a glass gob made of molten glass is supplied onto a lower die that is a backing gob forming die, and press forming is performed to the glass gob using the lower die and an upper die that is a counter gob forming die. More specifically, the glass substrate used in the magnetic disk is manufactured by the following method: a glass gob made of molten glass is supplied onto a lower die that is a backing gob forming die; press forming is performed to the glass gob to prepare a sheet glass material using the lower die and an upper die that is a counter gob forming die; and the sheet glass material is formed into am information recording medium glass substrate.
However, the surface irregularity of the sheet glass material formed in accordance with the conventional method is not sufficient for the surface irregularity accuracy of the principle surfaces for the high density of the magnetic recording and the fine magnetic recording information area.
For example, in forming the sheet glass material, a mold release agent is applied to the die surface in order to prevent the glass material from fusing to the die surfaces of the upper die and lower die. The surface roughness of the principal surface of the sheet glass material is increased because of the mold release agent. There is a large surface temperature difference between the upper die and the lower die, and the lower die to which the glass gob (a lump of the glass material) is supplied becomes high temperature. Because the surface temperature difference causes a temperature distribution in a thickness direction of the formed sheet glass material and in a plane of the plate, a shrinkage quantity of the sheet glass material that is taken out from the die and cooled also has a distribution in the thickness direction of the formed sheet glass material and in the plane of the plate. The sheet glass material is easy to warp, and therefore good flatness of the formed sheet glass material is not achieved.
With the sheet glass material obtained by the conventional press forming method, it is necessary that the flatness of the sheet glass material be improved up to the flatness required as a glass substrate for magnetic disk. Therefore, a grinding process is performed to the sheet glass material after the press forming, thereby improving the flatness of the glass material. However, performing the grinding process becomes an additional process in the manufacturing of the glass substrate for magnetic disk, and further performing the grinding process caused a “roll-off problem”.
That is, a machining allowance (ground quantity) is increased in the grinding process because the flatness is not so good with the conventional press forming method. For example, the machining allowance is about 200 μm with the conventional press forming method. When the machining allowance is increased in the grinding process, a deep crack is generated in the surface of the sheet glass material. Therefore, in a polishing process subsequent to the grinding process, the machining allowance (polishing quantity) is inevitably increased such that the deep crack is not left. Here, when the machining allowance is increased in the polishing process in which the loose abrasive grain and the resin polisher are used, the neighborhood in the outer circumferential edge portion is rounded in the principal surface of the sheet glass material to cause a “roll-off problem” of the edge portion. That is, because the neighborhood in the outer circumferential edge portion is rounded in the sheet glass material, a distance between the magnetic layer and the magnetic head in the neighborhood of the outer circumferential edge portion becomes larger than the floating distance of the magnetic head in another portion of the glass substrate when the magnetic disk is prepared using the sheet glass material as the glass substrate. The surface irregularity is generated because the neighborhood of the outer circumferential edge portion has the rounded shape. As a result, the recording and reading operations of the magnetic head are not precisely performed in the magnetic layer in the neighborhood of the outer circumferential edge portion. That is, the recordable and readable regions are reduced. The above is the “roll-off problem”.
Because the sheet glass material having the sufficient flatness is not obtained with the conventional press forming method, it takes a relatively longer time to perform the grinding process that is the post-process, and then the “roll-off problem” is generated by the grinding process.
On the other hand, with the floating method, the sheet glass material is obtained by continuously flowing the molten glass in a bath filled with molten metal such as tin. The molten glass is flown along a traveling direction in the bath to which an exact temperature operation is performed, and the belt-shaped glass ribbon is formed while finally adjusted to desired thickness and width. The sheet glass material that becomes the base of the glass substrate used in the magnetic disk is cut out from the glass ribbon. Because the tin surface in the bath is kept horizontal, the sheet glass material obtained by the floating method has the sufficiently high surface flatness.
In the other aspect, a predetermined impact resistance is required for the glass substrate used in the magnetic disk. Therefore, a chemically strengthening process is performed to the sheet glass material that becomes the base of the glass substrate in order to improve the impact resistance of the glass substrate.
The chemically strengthening process is performed as follows. A mixed solution of potassium nitrate and sodium sulfate is used as a chemically strengthening solution. For example, the chemically strengthening solution is heated to 300° C. to 400° C. For example, after the washed sheet glass material is preheated to 200° C. to 300° C., the sheet glass material is dipped in the chemically strengthening solution for 3 to 4 hours. Therefore, ion replacement with sodium ion and potassium ion occurs in the surface layer of the glass material to form a compressive stress layer. Accordingly, the crack that is possibly generated in the surface of the glass material hardly progresses to the inside of the glass material. The compressive stress layer formed through the chemically strengthening process has the thickness of about 50 to 200 μm.
However, when the chemically strengthening process is performed to the extremely-high-flatness sheet glass material obtained by the floating method, unfortunately a warp is generated in the sheet glass material. That is, in the sheet glass material obtained by the floating method, a tin diffusion layer having a thickness of about 10 to 50 μm is inevitably formed in one of the surfaces of the sheet glass material by tin used as the molten metal, and the tin diffusion layer is not formed in the other surface. When the chemically strengthening process is performed to the sheet glass material, the warp is generated to degrade the flatness because the compressive stress layers formed both the surfaces differs from each other by the presence or absence of tin diffusion layer. Therefore, when the chemically strengthening is performed, it is necessary that a grinding process for removing the tin diffusion layer be performed to the surface in which the tin diffusion layer is produced in the sheet glass material obtained by the floating method.
In summary, with the conventional press forming method, the grinding process is required because the sheet glass material having the sufficiently flatness is not obtained. With the floating method, although the sheet glass material having the sufficiently flatness is obtained, the grinding process is required to remove the tin diffusion layer of the material surface.
In the above regard, an object of the invention is to provide a method for efficiently manufacturing the glass substrate for magnetic disk having the good surface irregularity accuracy and impact resistance.